1. Field of the Invention
The present invention relates generally to phaselocked loops and, more particularly, to a novel sweep-to-lock circuit for a phase-locked loop of the type having an analog phase detector, which circuit causes rapid locking and provides automatic initiation of sweeping and resweeping, prevention of false locks, detection of correct lock, and halting of sweep at correct lock.
2. Background Art
Phase-locked loops are widely employed in electronic circuitry and are used, for example, in satellite communication systems, airborne navigation systems, and FM communication systems, providing the functions of, for example, phase detection, frequency modulation/demodulation, frequency division/multiplication (frequency synthesis), filtering, and voltage-controlled oscillator stabilization. The basic technique compares the frequency and phase of the incoming signal to the output of a voltage controlled oscillator (VCO). If the two signals differ in frequency and/or phase, an error voltage is generated and applied to the VCO, causing it to correct in the direction required for decreasing the difference. The correction procedure continues until lock is achieved, after which the VCO will continue to track the incoming signal as long as it remains within the bandwidth and operational frequency range of the loop.
Typically, such a circuit may employ a digital phase/frequency detector which ensures that loop lock is achieved no matter how far off frequency the system initially is. A disadvantage with such a detector, however, is that, although it is power efficient at low frequencies of hundreds of KHz or less, it consumes excessive power at higher frequencies of 10's of MHz or more and is unsuitable for applications such as in the transmit circuits of portable radios, for example. In such applications, the use of a frequency divider placed in front of the phase/frequency detector reduces the power problems somewhat, but the frequency divider generates a series of frequencies at multiples of the phase comparison frequency. The transmit frequency can leak into the frequency divider to produce frequencies which mix with multiples of the phase comparison frequency. The resulting frequency is often within the bandwidth of the loop, thus resulting in unwanted frequency modulation or the generation of spurious outputs.
To avoid the use of frequency dividers and digital phase/frequency detectors, analog phase detectors (typically mixers) are employed in higher frequency applications. Since this type of phase detector cannot automatically force the loop to lock, an auxiliary sweeping circuit is used to drive the transmit VCO to the proper transmit frequency.
The use of the analog mixer type of phase detector reduces the potential for generation of spurious frequencies greatly, but it adds some potential problems of its own. This is mainly due to the need to sweep the loop to lock. Minor potential problems are concerned with the possible added complexity of the sweep circuit. The major potential problems concern the phenomenon known as "false lock" and deciding when to sweep and when to terminate sweep. False lock refers to the situation in which the phase detector "sees" zero phase error, but the VCO is at the incorrect frequency. This can happen in two ways.
One type of false lock that is fairly easily prevented is that which occurs when the sweep circuit sweeps the VCO to a frequency which has a definite mathematical relationship to the correct frequency. In a simple tracking loop (illustrated later), this would be a harmonic of the correct frequency. A tracking loop is a loop which, when locked, provides an output frequency which equals the input frequency. In a translation loop (illustrated later), this false frequency would be the image of the correct frequency. A translation loop is a loop which, when locked, provides an output frequency which is offset from the input frequency by a constant difference over a range of operating frequencies. The image frequency is equal to the correct frequency plus or minus twice the frequency of phase comparison (the constant offset frequency referred to above), depending on whether the input frequency is above or below the output frequency when the loop is correctly locked. This type of false lock problem is solved by always sweeping in the proper direction. In the case of harmonic false locking (tracking loop) or where the image frequency (translation loop) is above the desired frequency, sweep always starts with the VCO at much less than the correct frequency, and the frequency increases with the sweep. This ensures that the VCO never sweeps through the false frequency, since it sweeps to the correct frequency first. The loop then locks and the sweep is stopped before the VCO reaches the false frequency. In the case of translation loops, if the image frequency is below the desired frequency, the VCO is swept in the other direction.
The other type of false lock is much more subtle and much more damaging than the harmonic or image false lock discussed above, as it is harder to detect. This type of false lock occurs when there is an interaction between the phase detector (which commonly has a sinusoidal or triangular phase characteristic in which the output polarity or slope alternates positive and negative for each 180 degrees of phase difference at the inputs), the VCO, and the entire loop filter's phase shift versus frequency characteristic. The "entire loop filter" includes the "classical" or "intended design" loop filter or integrator and any phase shift resulting from intermediate frequency amplifiers and filters, if present, and all extra, possibly unwanted but unavoidable poles which cause additional phase shift. As the VCO is sweeping and approaching the desired frequency, the phase detector begins to generate a beat frequency equal to the difference of the two frequencies at its inputs. This beat frequency appears as a ripple voltage at the phase detector output and frequency modulates the VCO. As the beat frequency approaches zero Hertz, or correct lock, the levels of the FM sidebands and their frequencies, which are offset from the VCO carrier frequency by multiples of the beat frequency, and the total phase shift around the loop create a situation where one of the FM sidebands is at the desired (correct) VCO frequency and is of sufficient amplitude that the loop locks on it.
In the above case, the loop is in a stable oscillatory mode where the phase detector "thinks" it is properly locked (i.e., sweeping stops), but it is only locked on a sideband caused by the beat frequency. If the loop's output signal (or the constant offset frequency in the case of translation loops) is frequency or phase modulated, it may also participate in the problem to some extent. This is not usually of serious concern if the loop is properly designed to prevent false locks when no modulation is present . This problem is exacerbated if the loop is made to have a very high gain (which is good for other reasons, such as a low steady-state phase error when the loop is locked), as the false lock only has to generate a very small DC component at the phase detector to hold the loop at the incorrect condition.
A further disadvantage of previously known phase-locked loops with analog detectors is that functions such as initiating sweeping, false lock avoidance, lock detection and sweeping termination, and re-sweep (if needed) are provided by auxiliary circuitry such as lock detectors, sweep generators, discriminators, and microprocessors. This adds complexity and cost to the system. In fact, the additional circuitry may be more complex and may consume more space, power, etc., than the basic loop itself.
Accordingly, it is a principal object of the present invention to provide a sweep-to-lock circuit for a phase-locked loop, which circuit is completely automatic and requires little auxiliary circuitry and no microprocessor inputs.
It is an additional object of the invention to provide such a circuit which rapidly achieves lock.
It is a further object of the invention to provide such as circuit which automatically provides for initiating sweeping, false lock avoidance, lock detection and sweeping termination, and re-sweep in the case that lock is lost for any reason.
lt is another object of the invention to provide such a circuit which is simple, economical, and consumes little power.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.